1. Field of the Invention
The present invention relates to a communication terminal device, a cellular radio communication system, and an information communication method, and for example, to those suitable for application to a communication terminal device, a cellular radio communication method, and an information radio communication system for mobile telephone.
2. Description of the Related Art
In a prior cellular radio communication system, an area in which communication services are provided is divided into cells of a predetermined size, and a base station is provided in each cell as a stationary station. Accordingly, this system allows a communication terminal device as a mobile station to communicate with a base station which seems to assure the best communication conditions. For this purpose, various systems and schemes have been developed for communication between communication terminal devices and base stations and among them, a typical one is the TDMA (Time Division Multiple Access) scheme.
The TDMA scheme is designed to form time slots by time-dividing a frequency channel and to transmit transmission signals at timings synchronized with time slots allocated to the home station. That is, this scheme allows a plurality of communication operations (so-called multiplex operation) to be performed at the same frequency channel so that available frequencies can be saved efficiently.
In the cellular radio communication system, a service area is divided into a plurality of cells and a base station is provided in each cell so that a mobile station which moves within the service area can communicate with the base station of any cell located within that service area. If the mobile station communicates within the service area of this cellular radio communication system, it selects such a base station that seems to assure the best communication conditions, performs the handoff process to switch the connection from the previous base station to the newly selected base station, and then communicates user information such as audio signals with the selected base station. This handoff process can also keep good communication conditions by switching base stations in sequence even when the mobile station moves from one cell to another.
To perform the handoff process, in fact, it is always required to know xe2x80x9cwhich base station should be connected to the mobile stationxe2x80x9d even during communication. In the cellular radio communication system, the mobile station usually receives control channel (hereinafter referred to as a control channel) signals transmitted by each base station and measures the power of each reception signal. Then the base station which transmits a signal of the highest power is determined as a possible base station for handoff. Accordingly, such determination of base stations for handoff is made principally by the mobile station.
However, the decision whether to actually perform the handoff process or not is made by the base station. Therefore, the handoff process consists of the steps of xe2x80x9cthe mobile station searching for a possible base station for handoffxe2x80x9d and xe2x80x9cthe possible base station performing the handoff process as a result of the searchxe2x80x9d. That is, the base station continuously monitors to determine whether the power received from the currently communicating communication terminal device is sufficient and if satisfactory communication quality cannot be achieved because of insufficient power, it is determined that the mobile station should be handed off from the current base station to another base station. Then the current base station is caused to perform the handoff process between the next base station which is notified of by the mobile station. On the contrary, the mobile station itself requests the handoff process when the power received from the base station decreases. However, the decision whether to perform the handoff process or not is finally made by the base station again.
As shown in FIG. 1, when time slots of transmission time and frequency for a time-divided control channel CCH are allocated uniquely to each base station in the cellular radio communication system according to the TDMA scheme, the mobile station can measure the reception conditions such as path loss (so-called transmission line loss) more easily than the received power described above by receiving the control channel CCH of a desired base station to measure the received power of control channel signals.
As shown in FIG. 2, when time slots of transmission time and frequency for a control channel CCH of each base station are not allocated uniquely to each base station in the cellular radio communication system according to the TDMA scheme, a signal wave (for example, traffic channel TCH1) for the traffic channel (so-called user information channel) TCH of another base station, appears as a strong interference wave on the same frequency channel as a control channel CCH1 of the base station which decided to receive it. In such a case, the power of control channel signals received from the base station cannot be measured accurately by measuring the received power of the selected control channel CCH1 because the received power contains noise components introduced by the interference wave and consequently, it should be very difficult to measure the path loss.
In view of the foregoing, an object of the present invention is to provide a communication terminal device, a cellular radio communication system, and an information communication method which can measure the reception conditions at a higher degree of accuracy by eliminating any influence of interference waves from received control channel signals.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.